Since the German Wikipedia only recently supplies web fonts, I added missing {{lang}} templates to a lot of articles during the last days, and I encountered several problems:
The thing I saw last is perhaps the easiest to fix: The mechanism is case sensitive. {{Lang|km|}} works, {{Lang|Km|}} doesn't.
Script suffixes as {{lang|bo-Tibt|}} or {{lang|am-Ethi}} do not work, so I had to change {{lang|bo-Tibt|}} to {{lang|bo|}}.
For the Ge'ez script, only {{lang|am}} and {{lang|ti}} work, {{lang|gez}} (Ge'ez language) and {{lang|tig}} (Tigre language) don't, so I left de:Ge’ez (Sprache) and de:Tigre (Sprache) without {{lang}}. Nor do all the smaller Ethiopian Semitic languages (for these, maybe the suffix -Ethi should be mandatory because some of them are also written in Arabic or hardly written at all).
For the Tibetan alphabet, only {{lang|bo|}} and {{lang|dz|}} work, not the other Category:Languages written in Tibetan script.